Gerard van Bussel

2D CFD Simulation of Dynamic Stall on a Vertical Axis Wind Turbine: Verification and Validation with PIV Measurements

After a decrease of interest in the 1990’s, the research on Vertical Axis Wind Turbines (VAWT) a has reappeared in the last years as a result of the its increasing application in the built environment, where VAWTs present several advantages over Horizontal Axis Wind Turbines (HAWT). The VAWT has an inherent unsteady aerodynamic behavior due to the variation of angle of attack with the angle of rotation µ, perceived velocity and consequentially Reynolds number. The phenomenon of dynamic stall is then an inherent efiect of the operation of a Vertical Axis Wind Turbine (VAWT) at low tip speed ratios (‚), having a signiflcant impact in both loads and power. The complexity of the unsteady aerodynamics of the VAWT makes it extremely attractive to be analyzed using Computational Fluid Dynamics (CFD) models, where an approximation of the continuity and momentum equations of the Navier-Stokes equations set is solved. The complexity of the problem and the need for new design approaches for VAWT for the built environment has driven the authors of this work to focus the research of CFD modeling of VAWT not in the perspective of creating one large academic model to test a particular situation, but to develop a work that would: † verify the sensitivity of the model to its grid reflnement (space and time), † evaluate the difierences between the difierent commonly used turbulence models (Laminar, Spalart i Allmaras and k i †), and † be evaluated using Particle Image Velocimetry (PIV) experimental data, thus determining the suitability of this data for model validation. The 2D model created represents the middle section of a single bladed VAWT with inflnite aspect ratio. The model simulates the experimental work b of ∞ow fleld measurement using Particle Image Velocimetry (PIV) by Sim~ao Ferreira et al 1 for a single bladed VAWT, for two difierent reference Reynolds numbers of Re = 52000 and Re = 70000 for three tip speed ratios: ‚ = 2;3;4. The results show the suitability of the PIV data for the validation of the model, the unsuitability of the application of a single turbulent model and the high sensitivity of the model to grid reflnement.

Wind Tunnel Hotwire Measurements, Flow Visualization and Thrust Measurement of a VAWT in Skew

The results of experimental research on the wake and induced flow around a vertical axis wind turbine (VAWT) in skew are presented. The previous research on VAWTs in skew is limited because this operation mode has only recently been found to be significant in the operation of VAWTs in the built environment. These results contain hotwire measurements of the incoming flow and wake of a VAWT in nonskewed and skewed flow. The high sampling rate of the hotwire data allows the effects of blade passing to be identified. Flow visualization of the tip vortices is also presented. Thrust measurements of the rotor were performed to understand the effect of skew on thrust variation and to compare with analytical predictions. Copyright

Measurement of Tip Vortex Paths in the Wake of a HAWT Under Yawed Flow Conditions

Tip vortex locations have been measured in the wake of a model rotor in both axial flow and yaw using quantitative flow visualization. For each setting, the axial force coefficient has been derived, as well, from measurements. The results agree well with those previously published on the Delft University of Technology model rotor. The main interest is to determine the tip vortex pitch, wake skew angle, wake expansion, and to physically interpret the data. The results also help to validate and construct models. The tip vortex location data complement the existing skewed wake velocity data from hot-wire anemometry, making it a valuable experimental database.

Measurement and Modelling of Tip Vortex Paths in the Wake of a HAWT under Yawed Flow Conditions

Tip vortex locations have been measured in the wake of a model rotor in both axial flow and yaw using quantitative flow visualisation. For each setting, the axial force coefficient has been derived as well from measurements. The results seem to agree well with those previously published on the Delft University of Technology model rotor. The main interest is to determine the tip vortex pitch, wake skew angle and wake expansion and to physically interpret the data. The results should also help to validate and construct models. The tip vortex locations data complement the existing skewed wake velocity data from hot-wire anemometry, making it a valuable experimental database.

On the use of velocity data for load estimation of a VAWT in dynamic stall

This paper focuses on evaluating the feasibility of estimating loads on vertical axis wind turbine blades in dynamic stall with velocity data acquired with Particle Image Velocimetry. The study uses numerical simulation data of a 2D Vertical Axis Wind Turbine in dynamic stall to verify sources of error and uncertainty and estimate the accuracy of the method. The integration of the forces from the velocity field overcomes the difficulties and limitations presented by pressure sensors for estimating the local section loads, but adds the difficulty in determining the correct velocity field and its time and spatial derivatives. The analysis also evaluates the use of phase-locked average data as an estimator of average loads.

Performance of a High Tip Speed Ratio H-Darrieus in the Skewed Flow on a Roof

Small wind turbines sited on a flat roof have good opportunities to become widespread. They operate in the accelerated wind above the roof and deliver the power where it is needed. Since the power produced offsets that which would otherwise be bought from the utility, they reduce energy demand and bills from the utility. Furthermore excess power can be sold back to the utility, thus producing income as well. Flow over a building separates at the roof leading edge at a certain angle. Wind turbines sited well above the roof thus operate in skewed flow. H-Darrieus operating at (flat) roofs just recently start to be at public interest, operation of an H-Darrieus in skewed flow is thus not discussed in literature until now. To examine this, a model of an H-Darrieus with high Tip Speed Ratio (λ) in skewed flow is developed. The model is based on multiple stream-tube theory: a combination of axial momentum and blade element theory on an actuator plate representation of the rotor, which is divided into multiple stream-tubes. The model shows that, for an H-Darrieus designed for skewed flow, the optimal power output in skewed flow can be up to two times the power output in normal - perpendicular to the H-Darrieus axis- flow. The spatial dimension of the H-Darrieus is responsible for this.Copyright

Effects of geometry and tip speed ratio on the HAWT blade's root flow

In this study, the effect of the parameters playing a role in the root flow behavior of HAWT are only partly understood. To better reveal the root flow properties, this study presents the progression of HAWT blade root flow at two different blade geometries and at two different tip speed ratios. The effects of the geometry and the tip speed ratio on the root flow behavior and on the evolution of the root flow features are investigated. This study aims to answer the following questions: (i) What are the effects of the blade geometry and tip speed ratio on the root flow behavior? (ii) How are the blade wake and the root vortex evolution affected by the change of these parameters? The analysis of the velocity fields shows that the radial flow behavior changes with different blade geometries but a remarkable difference in the radial flow behavior is not observed with the change of tip speed ratio. The formation of the wake is different at three test cases because of different loading that the blades are encountered. From the circulation distribution along the blades, while a strong root vortex can be observed in Blade 1, the bound vorticity along Blade 2 builds up gradually when moving outboard, and do not show a trace of a strong root vortex.

Estimating the Unsteady Angle of Attack from Blade Pressure Measurements on the NREL Phase VI Rotor in Yaw using a Free-Wake Vortex Model

The unsteady aerodynamic phenomena associated with yawed wind turbines are still poorly understood and are therefore challenging to predict accurately. The main issues concern the geometry of the skewed vortical wake formed behind the turbine, the unsteady flow field at the rotorplane induced by the vortical wake, as well as the aerodynamic effects of unsteady flow over the blade sections. Blade pressure measurements on a rotating blade of a yawed wind turbine can be very useful in obtaining further insight of yaw aerodynamics. However, in doing so, knowledge of the time-dependent angle of attack and induced velocity distributions at the rotoplane is an indispensable requirement. This paper presents a method to derive such distributions for the NREL phase VI turbine using a free-wake vortex model. The study considered different operating conditions in yaw that yielded both attached and separated flows over the blades. The derived free-wake geometry solutions are plotted together with the corresponding wake trailing and shed circulation distributions. These plots help investigate how the unsteady bound circulation formed at the blades is eventually convected into the wake. The derived results are helpful to develop more reliable aerodynamic models for wind turbine design codes.

Towards integral boundary layer modelling of vane-type vortex generators

This paper details an experimental investigation of submerged vane-type vortex generators (VGs) on a flat plate turbulent boundary layer inside a boundary layer wind tunnel. Spanwise planes at various positions downstream of the VG trailing edges are measured using stereoscopic particle image velocimetry (SPIV). The analysis focuses on the streamwise and spanwise development of the boundary layer, in light of the boundary layer integral parameters. The high spatial SPIV resolution enables an accurate resolution of the boundary layer with and without vortex generators. Through the study of the measured wake flow, insights towards an integral boundary layer modelling approach of vortex generators is gained through the verification of the vortex development properties as well as the net effect on the encompassing boundary layer.

Numerical simulation of transitional flow on a wind turbine airfoil with RANS-based transition model

ABSTRACT This paper presents a numerical investigation of transitional flow on the wind turbine airfoil DU91-W2-250 with chord-based Reynolds number Rec = 1.0 × 106. The Reynolds-averaged Navier–Stokes based transition model using laminar kinetic energy concept, namely the k − kL − ω model, is employed to resolve the boundary layer transition. Some ambiguities for this model are discussed and it is further implemented into OpenFOAM-2.1.1. The k − kL − ω model is first validated through the chosen wind turbine airfoil at the angle of attack (AoA) of 6.24° against wind tunnel measurement, where lift and drag coefficients, surface pressure distribution and transition location are compared. In order to reveal the transitional flow on the airfoil, the mean boundary layer profiles in three zones, namely the laminar, transitional and fully turbulent regimes, are investigated. Observation of flow at the transition location identifies the laminar separation bubble. The AoA effect on boundary layer transition over wind turbine airfoil is also studied. Increasing the AoA from −3° to 10°, the laminar separation bubble moves upstream and reduces in size, which is in close agreement with wind tunnel measurement.